1. Field of the Invention
The present invention relates to a semiconductor device having a trench structure and its manufacturing method. In particular, the invention relates to a semiconductor device and its manufacturing method which include a structure or a method of covering a trench formed in a semiconductor substrate with an insulating film.
2. Background Art
FIGS. 8 and 9 are schematic sectional views illustrating formation of an insulating film in a conventional semiconductor device manufacturing method. That is, FIGS. 8 and 9 show the main part in steps of forming a trench for, for instance, device isolation in a semiconductor substrate and covering the trench with an insulating film.
The manufacturing steps will be described below with reference to FIGS. 8 and 9. First, as shown in FIG. 8, the surface of a semiconductor substrate 1 is formed with a device isolation trench 2 by etching or the like. An insulating film 3 is formed by CVD or the like so as to cover the trench 2.
FIG. 8 shows the initial stage of the formation of the insulating film 3. Although the insulating film 3 is normally formed on the surface of the semiconductor substrate 1 excluding the trench 2, on the side surfaces 5 of the trench 2 it is formed at a relatively small thickness at a low density. In particular, the insulating film 3 is not grown sufficiently on the bottom 6 of the trench 2.
FIG. 9 shows the final stage of the formation of the insulating film 3, in which the insulating film 3 grows so as to close the surface opening 4 of the trench 2. In this state, the insulating film 3 is not sufficiently grown inside the trench 2, particularly on its bottom 6; there is a possibility that an insulator portion 7 having a low density or an interstice is formed.
As described above, in the insulating film forming method of the conventional semiconductor device manufacturing method, in the step of covering the trench 2 formed in the semiconductor substrate 1 with the insulating film 3, the surface opening 4 of the trench 2 is closed by the part of the insulating film 3 that has grown on the surface of the semiconductor substrate 1. Therefore, the low-density insulator portion 7 may be formed inside the trench 2.
More specifically, in the trench isolation structure for device isolation of a semiconductor device, when an insulating film is formed by an ordinary CVD method or a like technique in filling in a trench with the insulating film, it is difficult to satisfactorily form the insulating film in the deepest portion of the trench, possibly forming a low-density insulator portion. This may render the device isolation incomplete and hence adversely affect the electrical characteristics.